Golem is a userscript manager for Google Chrome and other browsers. It makes it very easy to create, update, and uninstall userscripts. All you need is to write a function, import the JavaScript API, and you can start to use it. For instance, let's write a simple function. This can be used for whatever purposes you might imagine. 

A very simple example is a basic calculator:

The source code is very easy to read, so you can understand how it works.

Another very simple example is a weather forecast:

A userscript that gives you the current temperature in °C and the forecast for the next 24 hours.

This userscript makes it very easy to get data from various websites.

The only thing missing in these examples is a reference to the source code. You can get this by installing the userscript into your browser,

and then you can go to "Script Editor" to find the function in the code.

Note that a user script can access all the APIs available in your browser, so it can also do something with your cookies or your location.